1. Field of the Invention
The present invention relates to feedforward amplifiers and more particularly to a method and system for aiding in the alignment procedure of feedforward amplifiers to achieve optimum performance.
In cable communication systems it is often necessary to transmit signals long distances over coaxial cables. The strength of the transmitted signals decreases in proportion to the length of the cable over which the signals are transmitted, necessitating amplification of the signals at repeated intervals along the cable to maintain adequate signal strength. The electronic amplifiers used to amplify the signals inherently distort the signal as they amplify it. Ideally, such distortion is to be eliminated or reduced to some acceptable minimum level.
Feedforward amplifiers are able to provide relatively distortion-free amplification by extracting from the amplified signal a signal component representative of the distortion introduced by the amplifier, phase inverting that component, and then recombining the phase inverted distortion component with the amplified signal. The phase inverted distortion component cancels the distortion component in the amplified signal, leaving a relatively distortion-free amplified signal for transmission along the cable. To avoid further distortion, the signals must coincide precisely in time when they are recombined. To ensure such coincidence, delay means are provided to delay the signals and thereby compensate for time delays which arise when the signals pass through the various electronic components in the amplifier. A particular feedforward amplifier is described in U.S. Pat. No. 4,472,725 to Robert M. Blumenkranz, an inventor of the present invention.
2. Description of the Prior Art
The alignment of feedforward amplifiers requires precise component matching and circuit balancing which can often take hours to accomplish. Various components such as capacitors and directional couplers must be adjusted to achieve the maximum performance from the amplifier. Typically, the adjustment procedure is accomplished by applying test signals to the amplifier in the form of an RF (radio frequency) sweep signal and viewing the amplifier response to the signal on an RF analyzer which includes a sweep display. Various components of the amplifier are adjusted while viewing the display in order to achieve optimum performance. For example, in the above-mentioned U.S. Pat. No. 4,472,725, a tuning procedure is described in which two different performance parameters are simultaneously displayed while making adjustments to the amplifier.
The difficulty in aligning feedforward amplifiers comes from the fact that the adjustment of various components can affect several different operational characteristics. Thus, by aligning the various components for optimum performance for a certain characteristic, other operational characteristics of the amplifier may be adversely affected. The testing of several of the performance characteristics is mutually exclusive, i.e., the test configuration is such that no additional tests can be simultaneously performed. As a result, the prior art alignment procedure involves performing adjustments while viewing displays of one or two operational characteristics, reconfiguring the test set up to display different operational characteristics and performing further alignment while viewing the displays of the additional operational characteristics. However, since each adjustment may affect many operational characteristics, watching displays of only one or two characteristics does not enable the technician to optimally adjust the amplifier.